My big thing is direct coupled SE triodes. I currently have a DC Darling, with a mu follower type 6SN7 input running at 9.6 mA.
I've been designing a full DHT amp, with a 26 driving a 45, to see what all that DHT talk is about. The 26 can push a good 6-8 mA of current, which I presume should be fine for the 45. However I recently found a pretty good deal on some NOS 30 triodes: NJ7P Tube Database Search
It looks really tempting for a couple of reasons; 1) it would be a cheap way to get my feet wet on the DHT front 2) with that low filament current I could use some serious chokes to get it properly clean.
However, I began to wonder about the current drive capabilities of the 30. Is 3.1 mA really sufficient to drive a 45? What about the 1626? How much current is enough? Is there a way to calculate it from some parameters?
I've been designing a full DHT amp, with a 26 driving a 45, to see what all that DHT talk is about. The 26 can push a good 6-8 mA of current, which I presume should be fine for the 45. However I recently found a pretty good deal on some NOS 30 triodes: NJ7P Tube Database Search
It looks really tempting for a couple of reasons; 1) it would be a cheap way to get my feet wet on the DHT front 2) with that low filament current I could use some serious chokes to get it properly clean.
However, I began to wonder about the current drive capabilities of the 30. Is 3.1 mA really sufficient to drive a 45? What about the 1626? How much current is enough? Is there a way to calculate it from some parameters?
You don't drive a class A amplifier with current. Well, at the high frequency end one does need to drive the load capacitance. Calculate the current needed for whatever high end you desire and make sure the tube draws at least that much.
What is DHT? Why do people insist on using acronyms to make themselves seem knowledgeable?
I am an experienced engineer and have, I think, a lot to offer technically. But I am not 'hip' to some of this jargon I read.
What is DHT? Why do people insist on using acronyms to make themselves seem knowledgeable?
I am an experienced engineer and have, I think, a lot to offer technically. But I am not 'hip' to some of this jargon I read.
bob91343:
I have found that the driver stage's current pushing capability greatly affects a power triodes performance - if you don't have enough current, you loose dynamics and body.
"to drive the load capacitance. Calculate the current needed for whatever high end you desire"
How?
PIM? A personal information manager (often referred to as a PIM tool or, more simply, a PIM)?
I have found that the driver stage's current pushing capability greatly affects a power triodes performance - if you don't have enough current, you loose dynamics and body.
"to drive the load capacitance. Calculate the current needed for whatever high end you desire"
How?
PIM? A personal information manager (often referred to as a PIM tool or, more simply, a PIM)?
Calculate the Miller capacitance of the triode, add a few pF for wiring stray capacitance, and then calculate the reactance at the highest frequency you wish to reproduce. This combined with the max output swing of the driver gives the max current that is needed.
Although not often done, there is no reason why a Class A output cannot in principle be used in the grid current regime. You would need a good driver, of course. The fact that grid current is usually associated with Class B (or AB1, AB2) does not mean that Class A cannot use grid current. The Class of an amplifier is defined by the output conduction as a proportion of the total cycle, not the input conduction. Otherwise, you would not be able to do Class A with BJTs!
Quote: The fact that grid current is usually associated with Class B (or AB1, AB2) does not mean that Class A cannot use grid current.
Wrong. Grid current is associated with class A2 and B2.
When, in a PP stage, one of the active elements is temporarily cut off during a full cycle, we have class B operation.
Pieter
Wrong. Grid current is associated with class A2 and B2.
When, in a PP stage, one of the active elements is temporarily cut off during a full cycle, we have class B operation.
Pieter
My point was that grid current can be associated with both Class A and B; it is not just Class B. I did not say that Class B requires grid current. Class A without grid current is A1; with grid current it is A2 - both are Class A. Similarly for B and AB (one could argue that pure Class B only exists in the tidy minds of those who write textbooks, as there is always a transition region).
A2 is not a separate class from A; it is one of the possible parts of A, the other being A1.
A2 is not a separate class from A; it is one of the possible parts of A, the other being A1.
This is also how I construe it. "Class A" being the more general term; no distinction based on grid regime is made until a digit is appended.
One time I saw someone use the suffix "3" to denote grid current being present over 360 degrees of the signal period.
BTW, the class designation of an amplifier is always to be made at full rated power. Just check the text in the old RCA tube manuals. Many people do not realize this, and then say such silly things as "This amp is Class A up to 5W".
Kenneth
I guess it depends on how the class is important to you. Personally, I don't care if my PP 300B amp is capable of putting out 15W to my speakers, when it operates at 1W or less 99% of the time. It would be a better argument that the 'official' class of an amplifier is largely irrelevant, and that the operation of the output devices at typical listening levels is more appropriate.
Thus Class A up to 5W is very relevant. Class is a description of operation at whatever output you care to define.
"Class A up to 5W" strictly means Class AB, but I think we know what people mean. The problem with this way of thinking is that too high a "Class A" power for an AB amplifier actually means more distortion at rated output as the quiescent bias is set too high. "Class A up to 5W" for a 15W amp would probably show poor engineering!
...and "3 dB of GNFB only!"
-- typical marketing numbers. Poor engineering for good marketing.
I understand the need for such a measure, but that's not how it's defined. Maybe those who define Class any way they see fit could invent another word? "Regime A" for example... It would help avoid ambiguous communication.
Ambiguous communication is the core of good marketing. Potential customers have to read what they already like. They "know" that class A is good, and feedback is bad. So "Up to 5W of class A" sounds appealing, as well as "3 dB of feedback only". You can even read sometimes such nonsenses like "Single ended class A stage".
I don't like it, though, when engineers have to trade-off quality for money...
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